Various picture coding methods are known from video technology. The invention provided herein for coding of a picture sequence complies with a picture coding method which is used for producing so-called sub-picture data units for DVD appliances. Such DVD appliances are already commercially available and each contain a sub-picture decoding device (or sub-picture decoder) which can process the said sub-picture data units. A description of the picture coding method used there can be found, for example, in the DVD Standard “DVD Specification for read-only disc”, part 3, video specification, Version 1.0, August 1996, Section 5.4.3 Sub-picture unit (SPU). A detailed description of the said sub-picture decoder devices can be found, for example, in EP-A-0 725 541.
According to the DVD Standard, the main application for sub-picture data units is, for example, the generation of subtitles on a television screen. The subtitles can then be overlaid anywhere in the picture, and they are generally overlaid at the lower edge of the picture. The bitmap data for in each case one subtitle are then stored in the sub-picture data unit SPU. In order to define the location of a subtitle on the screen, and to define a number of display characteristics, in particular the colour, transparency etc., simple graphics control instructions are specified in the said DVD Standard. Thus, with skilful programming, sub-picture data units can in principle also be used for other purposes, for example for control instructions, operator guidance menus, animation pictures and screen savers etc.
The essential parts of a sub-picture data unit according to the DVD Standard comprise a data field for the bitmap data of the text to be displayed or, in more general form, the object to be displayed, as well as a second part in which so-called display control instruction sequences are stored, which define the display location, the display time and the display characteristics of the stored object. Furthermore, a number of objects can also be stored successively in the data field for the bitmap data, and can each be called up at different times. If a number of objects are intended to be provided in a sub-picture data unit, then the DVD Standard specifies that a corresponding area must in each case be provided in the bitmap data field for each object. The bitmap data are then still run-length coded in order to achieve picture compression and to save memory space. While precautions are already taken in the said DVD Standard, as to how a text can be run through in the vertical direction in the overlaid sub-picture window using the existing display control instruction set, there is no information as to how it is possible to allow a text to be run through in the horizontal direction in the sub-picture window. Running through a text in the horizontal direction is often also referred to as running script, and can frequently be seen in some television programmes. One example which may be mentioned is the process of continuously running through stock market figures by television programme providers such as NTV.
Persons skilled in the art will very soon have the capability to use such running scripts in DVD appliances as well, utilizing the specified sub-picture data units, such that either a separate sub-picture data unit is stored for each individual frame of such a running script, or a sub-picture data unit would have to be provided for a number of phases of the running script, containing a correspondingly large number of bitmaps for the individual phases of the movement, which are then called up individually. As can be seen, these solutions are very memory-intensive. In any case, owing to the limitation in size of sub-picture data units to 53220 bytes, the second solution is feasible only for relatively short running scripts.